The present invention relates to a method of manufacturing golf balls having a good durability to impact, a soft feel, and a cover with excellent scuff resistance. The invention also relates to golf balls obtained by such a manufacturing method.
Conventional golf balls generally have a core which consists primarily of a diene rubber and a cover structure which is obtained by injection-molding a material made primarily of an ionomer resin over the core. In recent years, attempts have been made to use resin materials other than ionomer resins as the cover material in order to improve the feel of the ball on impact and the scuff resistance on approach shots.
For example, JP-A 11-104273 discloses a solid golf ball having a multilayer structure composed of a solid core and a two-layer cover encasing the core. The outer cover layer is composed primarily of a thermoplastic polyurethane elastomer. JP-A 10-219053 discloses the use of a polyamide elastomer as one component of a cover material for golf balls.
These golf balls have a good feel and a high scuff resistance. However, compared with ionomer resins, the cover material has an inferior compatibility with the diene rubber serving as the primary component of the core. As a result, on repeated impact, laminar separation tends to arise between the core surface and the adjoining cover layer.
In the golf ball industry, plasma treatment or corona discharge treatment is sometimes carried out as a means of increasing the adherence of, for example, a coat of paint. However, when such treatment is used to improve adhesion between the core surface and the adjoining cover layer, because the functional groups introduced by such treatment are relatively small groups such as hydroxyl groups or carboxyl groups, and because these functional groups are introduced onto a soft rubber surface, when the molten, high-temperature cover resin covers the core surface during injection molding, most of the functional groups thus introduced end up migrating from the core surface to the core interior, making it impossible to achieve the expected adhesion-improving effects. JP-A 5-317459 discloses the treatment of the core surface with an aqueous solution containing active chlorine (e.g., chlorine, concentrated hydrochloric acid, metal salts of hypochlorous acid), but such solutions are difficult to handle and bad for the environment, in addition to which they are not as effective as the aforementioned plasma treatment.
Another approach involves, as described in JP-A 11-253581, primer treatment in which a solution containing as a key ingredient a hot-melt resin is coated onto the surface to which the cover material is to be bonded. However, the high-temperature molten resin that flows over the core surface during injection molding dissolves and carries away the hot-melt resin serving as an ingredient in the primer, causing undesirable effects such as bleed-through to the parting line, which may adversely affect the durability and external appearance of the ball.
When the cover layer is formed by a heat compression molding or cast molding process using a thermoset polyurethane resin, the isocyanate included as an ingredient in the cover reacts with active hydrogen-containing functional groups derived from unsaturated carboxylic acids and/or metal salts thereof present in small amounts on the core surface. While this does improve adhesion with the core surface somewhat, the reaction takes too much time. As a result, this method has a far lower productivity than injection molding.
Accordingly, given the desire that exists for golf balls which are also endowed with a good feel and a good scuff resistance, there remains room for improvement in achieving a high durability to impact.